The present invention is directed to an improved apparatus for generating a reference signal representative of a reference braking condition in a brake control system. The apparatus of this invention is particularly useful in brake control systems such as antiskid systems and automatic braking systems, for example.
One type of commercially used brake control system includes means for generating a reference signal, such as a reference wheel velocity or a reference wheel acceleration, for example, which is representative of a reference braking condition. Generally, the brake control system also includes means for comparing the measured wheel signal with the reference signal, as well as means for modifying the braking action as necessary to bring the wheel signal into the desired relationship with the reference signal.
In a velocity based antiskid system, the reference signal may represent a desired wheel speed for an optimum level of braking effort and the modifying means may be designed to modify the braking effort as necessary to maintain the measured wheel speed substantially equal to the reference signal. Alternately, the reference signal may represent a reference wheel acceleration, and it may be compared with measured wheel acceleration to determine the appropriate level of braking effort. In the following description, these two types of brake control systems will be referred to as velocity-based systems and acceleration-based systems, respectively.
A further feature of many modern brake control systems is that the reference signal is generated either wholly or partly by measuring the rotation of a braked wheel. Often, the same wheel both provides information used to generate the reference signal and is braked by an amount controlled by the brake control system. This arrangement provides the important advantage that each braked wheel can be provided with an independent brake control system if desired. A high degree of redundancy and reliability is possible with this approach in that reference signals need not be shared by multiple wheels. Furthermore, variations in wheel or tire diameter, which may cause variations in measured wheel speed or acceleration, can be readily accommodated when a separate reference signal is maintained for each wheel.
There are, of course, difficulties in generating an accurate reference signal from measurements made on a braked wheel. Perhaps most important, a braked wheel is subject to variations in its rotational velocity as the braking effort is modified. At one extreme a braked wheel may rotate at a velocity comparable to that of an unbraked wheel; while at the other extreme, a braked wheel may be locked by the brakes. Intermediate conditions include slip velocities of zero to 100 percent of the unbraked wheel velocity. These variations in the wheel velocity as a function of braking effort complicate the generation of an accurate and reliable reference signal. An accurate reference signal may be crucial to proper brake control system operation, in that an inaccurate reference signal may result in a braking effort that is either greater or less than the optimum level.